High altitude platform control system

ABSTRACT

A method and system for controlling a movable appendage on an aerospace vehicle independent from the position of the aerospace vehicle. First and second beacons containing actual position information for the movable appendage are tracked and acquired by first and second beacon tracking sites. The beacon tracking sites report the azimuth and elevation data relating to the position of the movable appendage to a processor where a correction command is determined based on an error calculated from the position data and a desired position. The correction command is communicated to the movable appendage so that appropriate correction to the desired position can be made.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application is a divisional of U.S. patent application bearing Ser.No. 10/464,950, filed Jun. 18, 2003 now U.S. Pat. No. 6,929,220; whichis a continuation of Ser. No. 10/253,979, filed Sep. 23, 2002, nowissued as U.S. Pat. No. 6,631,871 on Oct. 14, 2003; which is acontinuation of Ser. No. 09/644,226, filed Aug. 21, 2000, entitled “HighAltitude Platform Control System”, inventor: David W. Lloyd, now issuedas U.S. Pat. No. 6,513,758, on Feb. 4, 2003; the entire contents of allapplications are incorporated herein by this reference.

TECHNICAL FIELD

The present invention relates generally to a movable platform orappendage as on an air or space vehicle, and more particularly to amovable platform or appendage that needs to be accurately controlled.

BACKGROUND ART

A reliable communications system depends heavily on the accurateposition control of movable platforms or appendages, such as antennas.These movable devices can be mounted on an air or space vehicle, or maybe part of a terrestrial system such as an electronically-steered phasedarray antenna.

In order to provide reliable service, the movable devices must bemaintained in certain predetermined, or fixed, positions. This isespecially important in air and space vehicles, which are constantlymoving and require certain movable devices to maintain relativepositions regardless of the motion of the vehicle.

There is a need for a low-cost reliable method of controlling a movabledevice such as a gimbaled communications platform attached to anunmanned aerospace vehicle.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the reliability of acommunications system. It is another object of the present invention toprovide a simple low-cost method for accurately controlling a movableappendage.

It is a further object of the present invention to provide a closed-loopcontrol system for a movable appendage. It is still a further object ofthe present invention to track the actual position of the device to becontrolled and position the device to a desired position.

In carrying out the above objects, the present invention provides aclosed-loop control system. Downlink beacons sent from the device beingcontrolled are received by tracking sites on the ground and track themovement of the device. The tracking sites report the device's positioninformation to a computer processor. The processor computes an errorthat is representative of the actual position of the device from thedesired position of the device to be controlled. The computer processorcomputes a command that is sent back to the device to be controlled tonull the error and drive the controlled device to the desired position.

These and other features of the present invention will be betterunderstood with regard to the following description, appended claims,and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a communications system that may employ thecontrol system of the present invention;

FIG. 2 is a block diagram of the control system of the presentinvention; and

FIG. 3 is a diagram illustrating the positioning for the ground-basedtracking sites.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1 there is shown a communications system 10 thatemploys a stratospheric aircraft 12 having a movable platform 14. Itshould be noted that while a stratospheric aircraft and a movableplatform are being illustrated and described with reference to thepresent invention herein, it is possible to apply the control system toother applications as well. For example, the present invention isapplicable to any unmanned aerospace vehicle, such as a satellite andany movable device such as an antenna.

Referring still to FIG. 1, the stratospheric aircraft 12 flies in afixed pattern while the communications platform 14 providescommunications service to and from ground based users 15. To provide theservice, the communications platform 14 must remain in a fixed position,in three axes, with respect to the ground. The fixed position of theplatform 14 is independent of the aircraft's motion.

Referring now to FIG. 2 there is shown a block diagram of astratospheric aircraft platform 14 and a closed-loop control system 20according to the present invention. The platform 14 has a gimbal driver16 and three-axis gimbal control 18.

The platform 14 includes a beam former 22 that allocates two beams asfirst and second downlink beacons 24 and 26. The downlink beacons 24 and26 are tracked by two beacon tracking sites 28 and 30 on the ground. Theplatform 14 for a stratospheric aircraft has beam-forming capability 22as part of its communications service. Therefore, there is no additionalcomplexity required to generate the first and second tracking beacons 24and 26 on board.

The tracking sites 28 and 30 are geographically located to provideoptimal geometry for the control system. The tracking sites 28 and 30are positioned with a predetermined distance between them in order toenhance the accuracy of the present invention. For example, according toone embodiment of the present invention, the tracking sites arepositioned as shown in FIG. 3, where the angle α between the nominalboresight of the tracking station's antenna 32 and 34 and the nominalnadir axis of the platform 14 is a minimum of five degrees, and theangular separation β between the first tracking site 28 and the secondtracking site 30 is approximately ninety degrees.

Referring again to FIG. 2, the tracking sites 28 and 30 have a gimbaledantenna 32 and 34 in order to acquire and track the downlink beacons 24and 26. Each tracking site 28 and 30 reports azimuth and elevation dataof the platform 14 to a ground based processor 36. The tracking sites 28and 30 can report data to the processor 36 by way of terrestrial links29. This allows the sites 28 and 30 to be optimally located and allowsthe processor 36 to be collocated with other centralized networkfacilities (not shown).

The processor 36 computes any errors between the actual position of theplatform 14 and the desired position of the platform 14. The processor36 then calculates the commands 38 necessary to null the errors. Thecommands 38 are transmitted to a receiver 40 where they are sent to thegimbal driver 16, which drives the gimbal 18 thereby adjusting theplatform 14 to the desired position.

The present invention does not require any information from the aircraftitself. For example, the position, attitude, rate, etc. of the aircraftare irrelevant to the control system of the present invention. Thissimplifies the aircraft/platform interface, which can reduce the cost ofthe overall flight system.

Additionally, it is not necessary to perform attitude control processingon board the platform. All of the processing can be done on the groundutilizing a low cost commercial computer. This significantly reduces theplatform weight and complexity, which ultimately reduces the platformcost. Also, because all of the attitude control processing is done onthe ground at the processor 38 instead of on the platform 14, softwaremaintenance is simplified and any modifications and upgrades to thesoftware are easier to perform.

The downlink beacons 24 and 26 provide position feedback. Therefore,there is no need for gyroscopes and attitude sensors on the platform.Nor is there a need for a resolver or other position feedback device onthe platform gimbal.

Typically in aerospace applications, uplink beacon tracking designsrequire beacon tracking arrays, solid-state modular assemblies, andtracking receivers on board the platform. None of this hardware isrequired for the present invention.

It is noted that the present invention may be used in a wide variety ofdifferent implementations encompassing many alternatives, modifications,and variations, which are apparent to those with ordinary skill in theart. Accordingly, the present invention is intended to embrace all suchalternatives, modifications and variations that fall within the spiritand scope of the appended claims.

1. A stratospheric platform communications system having beam formingcapability, said system comprising: a stratospheric aircraft; first andsecond downlink beacons generated by said beam forming capability, saidfirst and second downlink beacons having actual position information; afirst tracking site for acquiring and tracking said first downlinkbeacon said first tracking site having a first gimbaled antenna; asecond tracking site spaced a predetermined distance from said firsttracking site, said second tracking site for acquiring and tracking saidsecond downlink beacon, the second tracking site having a secondgimbaled antenna; said first and second tracking sites respectivelygenerating a first set of azimuth and elevation data relative totracking the first beacon with the first gimbaled antenna and a secondset of azimuth and elevation data relative to tracking the second beaconwith the second gimbaled antenna; and a processor in communication withsaid first and secord tracking sites and said stratospheric aircraft,said processor receiving said first set of azimuth and elevation dataand said second set of azimuth and elevation data from said first andsecond tracking sites to calculate an error between said actual positionand a desired position from said first set of azimuth and elevation dataand said second set of azimuth and elevation data, said error being usedby said processor to calculate a correction command that is sent to saidstratospheric aircraft.
 2. The system as claimed in claim 1 furthercomprising said first and second tracking sites being spaced 90 degreesfrom each other.
 3. The system as claimed in claim 1 wherein saidprocessor is located on the ground and communicates by way ofterrestrial links between said processor and said first and secondtracking sites.
 4. The system as claimed in claim 1 wherein said firstand second tracking sites are positioned with a predetermined distancetherebetween.
 5. The system as claimed in claim 1 further comprising afirm downlink terrestrial link coupling the first tracking site and theprocessor and a second terrestrial link coupling the second trackingsite and the processor, wherein said first and second beacon trackingsites and said processor are on the ground, and wherein said processorreceives the sad first set of azimuth and elevation data by way of thefirst terrestrial link and said second set of azinnih and elevation databy way of the second terrestrial link.
 6. The system as claimed in claim1 wherein said platform is an unmanned aerospace vehicle.
 7. Aclosed-loop control method for a movable platform, comprising:generating first and second beacons on-board the movable platform;acquiring and tracking the first and second beacons respectively usingfirst and second beacon tracking sites, said first beacon tracking sitehaving a first gimbaled antenna and the second beacon tracking sitehaving a second gimbaled antenna; with said first and second trackingsites respectively, generating a first set of azimuth and elevation datarelative to tracking the first beacon with the first gimbaled antennaand a second set of azimuth and elevation data relative to tracking thesecond beacon with the second gimbaled antenna; transmitting the firstset of azimuth and elevation data relative to tracking the first beaconwith the first gimbaled antenna and the second set of azimuth andelevation data relative to tracking the second beacon with the secondgimbaled antenna from the first and second beacon tracking sites to aprocessor, computing, in the processor. any errors between the actualposition of the movable platform and a desired position of the movableplatform using the first set of azimuth and elevation data relative totracking the first beacon with the first gimbaled antenna and the secondset of azimuth and elevation data relative to tracking the second beaconwith the second gimbaled antenna; generating commands necessary to nullthe errors, and move the movable platform to the desired position, andtransmitting the commands from the processor to the moveable platform.8. The method as claimed in claim
 7. further comprising: receiving thecommands from the processor, and adjusting the position of the movableplatform.
 9. The method as claimed in claim 8, further comprisingpositioning said first and second beacon tracking sites with apredetermined distance therebetween on the ground.
 10. The method asclaimed in claim 9, wherein the angle .alpha. between a nominalboresight of antennas associated with the first and second beacontracking sites and a nominal nadir axis of the movable platform is aminimum of at least about five degrees, and the angular separationbetween the first and second beacon tracking sites is approximatelyninety degrees.
 11. The method as claimed in claim 9, wherein theprocessor is ground-based and transmitting position data from the firstand second beacon tracking sites to the processor comprises transmittingthe position data by way of terrestrial links.